Scripts with empty description fail on Oracle

[juliahayward]

To reproduce:

• create a script V1.sql (contents are irrelevant, as long as they succeed - eg. SELECT 1 FROM DUAL)
• migrate

On Oracle - and presumably other databases that conflate null and empty-string - this will fail as Flyway tries to insert a null description into the history table.

[juliahayward]

Added to this, when you INSERT empty-string into a Sybase table, it converts it to a single space. This means that when we validate, the saved description no longer matches the original.